How do you preserve or embalm food as an actual decor?

You don't. The problem is that it discolors over time.
Get yourself wood, plaster, wax, or plastic replicas.
Be careful, though. I'm pretty sure every one of us has accidentally bitten into a wax apple at one time or another.

Not all food could be preserved.
Some you can preserve by drying, such as fruit or meat.
Some you can preserve by coating it in a sealant (root vegetables, etc.).

You can encase it in transparent plexiglass plastic.